Continued advances in the design of electronic devices for data processing and communications systems are placing rigorous demands on electrical connectors. Specifically, electrical connectors having higher densities and pin counts are needed for design advances which increase integration of solid state devices and which increase the speed of data processing and communication. Designing connectors to have higher densities and higher pin counts requires careful consideration of the problems which result from decreasing the distance between contacts. Primarily, as the distance between contacts decreases, the likelihood of undesirable electrical cross talk between contacts increases.
Density and pin count are often viewed interchangeably, but there are important differences. Density refers to the number of signal contacts provided per unit length. In contrast, the number of contact elements that can reasonably withstand the mating and unmating forces is referred to as the pin count.
As more functions become integrated on semiconductor chips or on flexible circuit substrates and more chips are provided on printed circuit boards (PCBs), each PCB or flexible circuit must provide more inputs and outputs (I/Os). The demand for more I/Os directly translates to a demand for greater density.
Moreover as signal frequency increases, which will occur as speed of data processing and communication increases, traditional approaches to connector design are less applicable. The connectors used in high-speed board-to-board, board-to-cable and cable-to-cable communications may be treated for design purposes like transmission lines in which crosstalk and noise become significant concerns. Indeed, the electrical performance of high-speed board-to-board, board-to-cable and cable-to-cable communications is dependent upon the amount of crosstalk and noise introduced at the connector interface.
As was recognized in U.S. Pat. No. 4,824,383--Lemke, incorporated herein by reference, an important connector design consideration is the provision of an electrical connection while avoiding degradation of component performance. Prior to this patent, connector designs had been proposed in which a ground plane and alternating ground contacts together with shielding extensions were introduced to minimize electrical discontinuities, i.e., crosstalk and noise. While performance was controlled in such prior devices, density was limited.
U.S. Pat. No. 4,824,383 proposed designs for plug and receptacle connectors for multiple conductor cables or multiple trace substrates. In such designs individual contact elements or groups of contact elements were electrically isolated to prevent or minimize crosstalk and signal degradation. In the individually isolated design, a conductive base plate was provided with a number of walls arranged in side-by-side relationship, thereby defining a number of channels. A contact support member formed from electrical insulating material was designed to have a number of fingers, wherein a finger was positioned within each channel. Each finger of the contact support member supported an individual contact element.
Although, the connectors disclosed in U.S. Pat. No. 4,824,383 increased contact element density, industry driven density demands continued to grow. U.S. Pat. Nos. 5,057,028--Lemke et al. and 5,169,324--Lemke et al. (now U.S. Pat. No. Re. 35.508), all incorporated herein by reference, disclose two row plug and receptacle connectors for attachment to printed circuit boards (PCBs), which provided increased density. Although, this plug and receptacle system provided higher contact density, electrical isolation was achieved primarily between sets of contacts by continuous metal structures rather than between individual contacts.
In an attempt to provide isolation between individual contacts, various design schemes have been proposed. These design schemes can be generally categorized as a coaxial structure (a single contact fully surrounded by a conductor), a pseudo coaxial structure such as a twinax structure (dual contacts surrounded by a conductor), as a microstrip structure (a number of contacts provided on one side of a single ground plane), and as a stripline structure (a number of contacts sandwiched between two ground planes).
U.S. Pat. Nos. 4,846,727, 5,046,960, 5,066,236, 5,104, 341, 5,496,183, 5,342,211 and 5,286,212 disclose various forms of stripline structures incorporated into a plug and receptacle system. Generally, however, these systems can be described as providing columns of contact elements having conductive plates disposed between each column. The connectors are designed so that the plug and receptacle ground plates contact one another. Each row of receptacle contact elements are molded into a frame of dielectric material. The overall receptacle assembly, thus includes, a housing to which the ground plates and dielectric frames are attached in alternating layers.
Particular reference is made in U.S. Pat. No. 5,046,960, which indicates that such connectors may not be desirable for high density applications due to the amount of dielectric material between each contact. This patent suggests that if one were to reduce the amount of dielectric material, the electrical characteristics of the connector, particularly impedance characteristics, would also be changed. It is stated that a desire would be to have a connector which provides a more dense array of contact members while maintaining the electrical characteristics associated with less dense connectors. Electrical characteristics are said to be achieved, in part, by the provision of air reservoirs immediately surrounding portions of the grounded, continuous conductive plates. Outer shields are also disclosed for surrounding the receptacle exterior. One of the problems of this system, however, is that due to the continuous structure of the conductive plates and the presence of dielectric material between the conductive plates, the speed by which signals may pass through the connector is being limited.
The present invention concerns, in part, a modification to the coaxial and twinax isolation schemes described thus far. It has been found that satisfactory isolation can be achieved by selecting particular contact elements in an array as signal and ground contacts. One such example is where a central contact in an array is selected for the transmission of a potential cross talk producing signal and the surrounding contacts are all connected to ground. Such contact element patterns are suggested in U.S. Pat. Nos. 5,174,770, 5,197,893 and 5,525,067.
One of the problems with the above described connector systems is that the contact element density remains insufficient for certain applications. Moreover, where the ground plate is a continuous metal structure, the capacitance or impedance characteristics of such a structure become more significant as speed increases. Increasing signal speed, as used herein, means decreasing rise time. When rise time decreases to a point where it is smaller than the propagation delay time characteristic of the connector structure, unwanted cross talk will occur.
Consequently, a need still exists for a connector system which maximizes the number of contact elements available for ground/signal assignment while minimizing cross talk.